Arenal Volcano is a prominent, conical landmark located in the northern region of Costa Rica, near the town of La Fortuna. Standing at 1,633 meters (5,358 feet) high, the volcano is one of the country’s most recognized and geologically young features, estimated to be less than 7,500 years old. Arenal Volcano is classified as a stratovolcano, also known as a composite volcano. It is part of the Cordillera de Tilarán mountain range and was historically one of the most active volcanoes in the world until its activity declined after 2010.
Arenal’s Primary Classification
A stratovolcano is characterized by its distinct, steep-sided, and often symmetrical cone shape. This iconic profile is a result of the volcano being built up over thousands of years from alternating layers of cooled, hardened lava flows and fragments of rock, ash, and cinders (tephra). The term “composite volcano” is frequently used because the structure is a complex composition of these various volcanic materials.
The highly explosive nature of stratovolcanoes is directly tied to the composition of their magma, which is typically andesitic or dacitic, indicating high to intermediate levels of silica. This high silica content makes the magma thick and viscous; it does not flow easily and cools quickly. As this thick magma rises, it traps large amounts of gas, building up immense pressure beneath the surface.
When the pressure exceeds the strength of the overlying rock, the result is an explosive eruption, which deposits ash and pyroclastic material close to the vent. Viscous lava flows that escape solidify quickly, stacking up steeply around the central vent and contributing to the volcano’s conical appearance. Arenal’s rock is primarily basaltic andesite, a type that perfectly aligns with this explosive and effusive formation process.
Geological Setting and Structure
Arenal’s existence as a stratovolcano is a direct consequence of its location within a specific tectonic environment. It is situated in the Central American Volcanic Arc, a chain of volcanoes formed by subduction. This occurs where the denser Cocos Plate slides underneath the Caribbean Plate.
As the Cocos Plate descends into the Earth’s mantle, heat and pressure cause it to release water and other volatiles. This lowers the melting point of the overlying mantle rock, generating magma. The resulting magma is rich in silica and gases, the exact composition necessary to fuel the explosive eruptions that build a stratovolcano.
Arenal rises sharply from the Cordillera de Tilarán mountain range, showcasing this geological history. Prior to its most significant modern eruption, it possessed a historically near-perfect conical shape. The volcano’s flanks reveal the alternating layers of its construction.
History of Activity and Current State
Arenal was long considered dormant before its sudden and violent reawakening on July 29, 1968. This major eruption, which occurred after centuries of quiescence, was catastrophic, destroying three small villages and killing 87 people. The explosion fundamentally changed the volcano’s structure, creating new craters and redefining its status as one of the most active in the world.
Following the 1968 event, Arenal entered a period of almost continuous activity that lasted for 42 years. For decades, the volcano was famous for near-constant emissions of ash, lava flows, and pyroclastic avalanches. This activity made it a major spectacle for visitors and a subject of study for seismologists.
The sustained eruptive phase eventually slowed down, with the last confirmed major eruption occurring in 2010. Since then, Arenal has been in a quiescent or “resting phase” with minimal volcanic activity. However, it is still classified by geologists as an active volcano that retains the potential for future eruptions. The volcano is closely monitored by the Costa Rican Volcanological and Seismological Observatory to ensure the safety of the surrounding communities and visitors.